This is a very simple easter egg that can give you a weapon. In my three times I've gotten an MG08/15, and 2 HAMRs. I have no idea if it only gives lmgs, but this is a simple easter egg to get you a good gun for free.

Step 1: Grab the plunger
The plunger is located outside of gen 4, in the power facility area. On the left side, behind some boxes, is the plunger.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1177702944


Step 2: Unclog the toilet
Inside of the bathrooms, on the side of mule kick, is a toilet. This toilet will allow you to unclog it if you have the plunger. Do this, and it will spit out a key.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1177703169

Step 3: Unlock the box
Immediately take this key out the door from the bathroom (to the grassy side, not the road side) and go straight. At the end of this area next to a cargo crate is a big box. Use the key on the box, and you get a free weapon.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1177703416

I've found this to be a really good easter egg because a skilled player won't need to even buy a gun before they do this easter egg, meaning that they have a powerful gun with only spending money on doors.